JP2000305119A - Optical device for wavelength conversion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the moisture resistance of a wavelength conversion optical device and to decrease the light reflection loss occurring at the entering and leaving faces. SOLUTION: This optical device is obtd. by joining planer optical devices c1, c2 having little moisture absorption to the entering face and exiting face of a CLBO crystal nc for wavelength conversion, respectively, into an optical contact state. An antireflection film Tf1 corresponding to the wavelength of the incident light is applied on the surface of the planer optical device c1 joined to the entrance side of the light of the CLBO crystal nc, while an antireflection film Tf2 corresponding to at least the wavelength of the wavelength converted light is applied on the surface of the planer optical device c2 joined to the exit side of the light of the CLBO crystal nc. Or, instead of applying the antireflection film Tf1, inclined faces making the Brewster angles corresponding to the incident light and outgoing light may be formed on the planer optical devices c1, c2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical element for wavelength conversion, and more particularly, to joining a cover material to a light entrance surface and a light exit surface of a nonlinear optical crystal to improve moisture proofness and reduce reflection loss. The present invention relates to an optical element for wavelength conversion.

[0002]

2. Description of the Related Art Nd: YAG, Yb: YAG, Nd: Y
An apparatus for converting the wavelength of a basic laser beam output from a solid-state laser using LF, Nd: YVO ₄ or the like to generate ultraviolet rays has been put to practical use. In these devices,
A CLBO crystal can be considered as a non-linear optical crystal used for the generation of the fourth harmonic, the generation of the fifth harmonic, and the generation of the sum frequency light of 300 nm or less. The CLBO crystal has a relatively large nonlinear constant, a small absorption at 190 nm to 300 nm, and satisfies the phase matching condition for the fourth harmonic generation of the fundamental laser light oscillating at around 1000 nm. Promising as a nonlinear optical crystal for generating a wavelength.

[0003]

However, the CLBO crystal has a drawback that it easily adsorbs moisture contained in air from the surface. The surface that has absorbed moisture has reduced durability against laser light. In addition, the part that adsorbed moisture expands,
Since the crystal is distorted, the conversion efficiency is reduced, the wavefront of the emitted light is distorted, and the like, which causes a practical problem.
For this reason, at present, a method is employed in which annealing is performed at a temperature of 150 ° C. or more before use to evaporate water before use.

[0004] However, even after annealing, if the temperature is lowered, moisture adsorption occurs, and care must be taken in the storage method. Since no good storage method has been found at present, annealing is performed immediately before use. Since the surface is liable to change in this way, it is difficult to deposit an antireflection film on the surface, and at present, the antireflection film is used without being deposited on the surface. Therefore, a loss of surface reflection (a reflection loss of 5% for each of the incident side and the output side) occurs during wavelength conversion.

[0005] In addition, since the reaction with moisture in the air occurs even when used at a high temperature, a method is used in which a CLBO crystal is placed in a sealed cell, and oxygen gas or the like from which moisture has been removed is sealed in the cell. Is also considered. In this case, since the window of the cell is required on the entrance side and the exit side, the above-described reflection loss is further increased. The present invention has been made in view of the above circumstances, and an object of the present invention is to improve the moisture resistance of the above-described wavelength conversion optical element and to reduce the reflection loss on the light entrance / exit surface. It is.

[0006]

An optical element which is hardly affected by moisture (hereinafter referred to as a cover material) is bonded to both ends of a CLBO crystal by an optical contact. By this method, it is possible to realize the CLBO crystal moisture proof, which is one of the conventional problems. The cover material is required to satisfy the following requirements. (1) Less susceptible to moisture. (2) The transmittance of light of the used wavelength is good. (3) The refractive index is close to that of the CLBO crystal. Specific materials include, for example, optical crystals such as CaF ₂ , quartz, lithium tetraborate, and MgF _2, and optical glasses such as BK7 and quartz. In addition, by adopting an anti-reflection film on the side other than the joining surface of the cover material, or by using a Brewster-cut cover material, anti-reflection can be effectively performed, and wavelength conversion, which has been a problem in the past, has been a problem. Reflection loss on the surface of the optical element can be reduced.

Here, in general, reflection occurs at a joint surface between two substances. This reflection is represented by R = (n1-n2) ² / (n1) where n1 and n2 are the refractive indexes of the two materials.
+ N2) ² . For example, CLBO crystal, CaF
₂ at 266 nm was 1.546 each.
166 and 1.4626209, so R = 0.0
0078, and the reflection at the joint surface between the CLBO crystal and CaF ₂ is extremely small. Therefore, if CaF ₂ is used as the cover material of the CLBO crystal, the reflection loss at the interface becomes negligible. Therefore, if an anti-reflection film is deposited on CaF ₂ , the reflection on the cover material can be reduced to about 0.3% or less, and the reflection as a whole can be prevented. In addition, by processing one or both of the CaF ₂ on the incident side and the exit side at a Brewster angle, the reflection can be made almost zero.

Based on the above, the present invention solves the above-mentioned problem as follows. (1) A flat optical element made of a material having low hygroscopicity is bonded to the light incident surface and the light output surface of the CLBO crystal for wavelength conversion by an optical contact, respectively.
An antireflection film corresponding to the wavelength of the incident light is provided on the surface of the flat optical element bonded to the light incident side of the crystal.
An antireflection film corresponding to at least the wavelength of the wavelength-converted light is provided on the surface of the flat optical element bonded to the light emission side of the BO crystal. (2) A flat optical element made of a material having low hygroscopicity is bonded to the light incident surface of the CLBO crystal for wavelength conversion by optical contact, and an antireflection film corresponding to the wavelength of the incident light is formed on the surface of the flat optical element. An optical element made of a material having low hygroscopicity and having a Brewster angle corresponding to the wavelength and the polarization direction of the wavelength-converted light is bonded to the light emitting surface of the CLBO crystal by an optical contact. (3) An optical element made of a material having low hygroscopicity and having a Brewster angle corresponding to the wavelength and polarization direction of the incident light is bonded to the light incident surface of the wavelength conversion CLBO crystal by an optical contact. An optical element made of a material having low hygroscopicity and having a Brewster angle corresponding to the wavelength and polarization direction of the wavelength-converted light is bonded to the light emitting surface of the crystal by optical contact.

[0009]

Embodiments of the present invention will be described below. FIG. 1 is a diagram showing a configuration of a wavelength conversion optical element according to a first embodiment of the present invention. This figure shows that the plate-like cover materials c1 and c2 formed of, for example, the above-mentioned CaF ₂ are bonded to both ends of the CLBO crystal nc.
The CLBO crystal nc and the cover materials c1 and c2 are bonded by the optical contact oc, and the reflection on the bonding surface is extremely small. The anti-reflection film Tf1,
Tf2 is applied, and it is designed so that the reflection loss on these surfaces for the wavelength to be used is 0.3% or less.

FIG. 2 shows the wavelength conversion optical element,
FIG. 3 is a diagram illustrating a case where second harmonic light [hereinafter referred to as a second harmonic (2ω)] of a fundamental light having a wavelength ω [hereinafter referred to as a fundamental wave (ω)] is obtained. As shown in the figure, the second harmonic (2
When the fundamental wave (ω) is incident on the CLBO crystal nc during the wavelength conversion to ω), the second harmonic (2ω) that is the second harmonic from the CLBO crystal nc and the fundamental wave remaining without wavelength conversion (Ω) is emitted. Therefore, in this case, an anti-reflection film Tf1 corresponding to the fundamental wave (ω) is provided on the cover material c1 on the light incident side, and a second harmonic (2ω) is supported on the light emission side cover material c2. Antireflection film T
Attach f2. Here, since the fundamental wave (ω) emitted from the CLBO crystal nc is not required, the antireflection film Tf2 is
It is not necessary to correspond to the fundamental wave (ω) emitted from the crystal.

FIG. 3 shows the wavelength conversion optical element,
It is a figure explaining the case where the 3rd harmonic (3ω) of a fundamental wave (ω) is obtained. When obtaining the third harmonic (3ω) from the fundamental wave (ω), as shown in FIG.
A fundamental wave (ω) is incident on c1 to obtain a second harmonic (2ω) of the fundamental wave (ω). Next, the second harmonic (2ω)
The fundamental wave (ω) remaining without being converted is incident on the second CLBO crystal nc2, and the third harmonic (3ω) is emitted from the second CLBO crystal nc2. Also in this case,
The fundamental wave (ω) and the second harmonic (2ω) remaining without being converted are emitted from the second CLBO crystal nc2.

Therefore, in this case, the anti-reflection film T
Those corresponding to the following wavelengths are used as f1 to Tf4.
An antireflection film Tf1 corresponding to the fundamental wave (ω) is provided on the cover material c1 on the light incident side of the first CLBO crystal nc1. Further, an antireflection film Tf2 corresponding to the fundamental wave (ω) and the second harmonic (2ω) is provided on the cover material c2 on the light incident side of the first CLBO crystal nc1. An antireflection film Tf3 corresponding to the fundamental wave (ω) and the second harmonic (2ω) is provided on the cover material c3 on the light incident side of the second CLBO crystal nc2. Further, an antireflection film Tf4 corresponding to the third harmonic (3ω) is provided on the cover material c4 on the light emission side of the second CLBO crystal nc2.

As described above, when there are a plurality of incident lights, an antireflection film corresponding to all the incident lights necessary for wavelength conversion in the crystal is provided on the light incident side. On the other hand, on the light emitting side, an antireflection film corresponding to light having a wavelength required in the subsequent stage is provided. For example, in the case of FIG. 2, an antireflection film corresponding to only the wavelength-converted light (the second harmonic (2ω)) is provided on the light emission side. In the case of FIG. 3, however, the first CLBO crystal nc1 is provided. In the light emission side of the wavelength conversion light (second harmonic (2
ω)) and an antireflection film corresponding to both the wavelength-unconverted fundamental wave (ω).

FIG. 4 is a diagram showing a configuration of a wavelength conversion optical element according to a second embodiment of the present invention. In the figure, a cover material c1 having a flat plate shape on the light incident side and an antireflection film Tf1 provided on the s1 surface is attached by an optical contact oc.
A cover material c having an inclined surface formed on the light emitting side so as to have a Brewster angle corresponding to the wavelength of the emitted light.
2 shows a case where the optical disc 2 is attached by an optical contact oc. Here, the Brewster angle means that when there is a boundary surface between two materials having different refractive indexes (in this case, a cover material and air), the light falls on a plane determined by the normal line of the boundary surface and the light traveling direction. Means the angle at which the reflection becomes 0 when there is polarized light.

In FIG. 4, when the angle θ is θ = tan ⁻¹ nc2
Is the Brewster angle. Note that nc
2 is the refractive index of the cover material c2. Light having a polarization plane in the plane of the paper, which is incident on the cover material c2 cut at such an angle from the inside of the cover material, has a reflectance of 0 in theory even if the anti-reflection film is not provided on the emission surface s2. It can be. In general, when wavelength conversion is performed using the nonlinear effect, the converted and emitted light has a fixed polarization direction, and thus the cover material may be cut in accordance with this direction. The refractive index and Brewster angle with respect to the wavelength of CaF ₂ are as shown in Table 1. When CaF ₂ is used as a cover material and the second to fourth harmonics (wavelength 266 nm) of a YAG laser are generated, ,
The Brewster angle may be set to θ = 55.63 °.

[0016]

[Table 1]

The shape of the inclined surface forming the Brewster angle is determined according to the wavelength and the polarization direction.
In the case where the polarization direction is a direction perpendicular to the paper surface, the inclined surface is a surface obtained by rotating the inclined surface shown in the figure by 90 ° with the traveling direction of light passing through the cover material c2 as a rotation axis. There is a need. When the second harmonic is obtained from the fundamental wave, the polarization direction of the incident fundamental wave (ω) and the fundamental wave (ω) whose wavelength has not been converted, and the polarization direction of the second harmonic (2ω) whose wavelength has been converted. Are different from each other. Therefore, when the second harmonic is obtained using the wavelength conversion optical element shown in FIG. 4, the antireflection film Tf1 applied to the cover material c1 corresponds to the fundamental wave (ω), and the cover material c2
Is required to have a Brewster angle corresponding to the polarization direction of the wavelength-converted second harmonic (2ω). As a matter of course, the cover material c2
Is the above shape, the second harmonic is the wavelength
The fundamental wave (ω) which is different from the polarization direction and not wavelength converted
Can not respond.

FIG. 5 is a diagram showing a configuration of a wavelength conversion optical element according to a third embodiment of the present invention. The figure shows a case where cover materials c1 and c2 having inclined surfaces formed to have a Brewster angle with respect to incident light and outgoing light are adhered to a CLBO crystal nc by optical contacts oc. FIG. 1A is a top view, and FIG. 1B is a side view. Note that yz and xy shown in FIG.
3B shows coordinate axes in (b), a yz plane, and x-
The y planes are planes orthogonal to each other. In this case, since the polarization direction and wavelength of the incident light and the emitted light are different, the mounting direction and the cut angle of the incident side cover material c1 are different from the mounting direction and the cut angle of the output side cover material c2. ing.

Here, when wavelength conversion is performed, the following Ty is used.
PeI and Type II phase matching is used. (1) Type I: ooe crystal The polarization direction of the incident light is all o-polarization, and the polarization direction of the wavelength-converted light is e-polarization (the polarization directions of o-polarization and e-polarization are orthogonal). (2) Type II: eoe crystal One of the polarization directions of the incident light is e-polarization, the other is o-polarization, and the polarization direction of the wavelength-converted light is e-polarization. For example, the phase of Type I from the second harmonic (wavelength 532 nm) of the light emitted by the YAG laser light source When the fourth harmonic (wavelength 266 nm) is obtained by wavelength conversion by matching, the polarization direction of the incident light is o and the polarization direction of the wavelength-converted light is e as described above. The polarization directions are orthogonal to each other.

Therefore, in FIG. 5, when the c-axis of the CLBO crystal nc is on the xy plane, the cut surface of the cover material c1 on the incident side is perpendicular to the xy plane and (90 ° -θ1), and the polarization direction of the incident light is set to be on the xy plane. θ1 is the Brewster angle according to the wavelength of the incident light, and θ1 = tan
^-1 [n (532 nm)]. In addition, the cut surface of the cover material c2 on the emission side is perpendicular to the yz plane and forms an angle of (90 ° -θ2) with the optical axis of the emitted light, and the polarization direction of the emitted light is on the yz plane. It is in. θ2 is a Brewster angle adjusted to the wavelength of the emitted light, and θ2 = tan ⁻¹ (n (266n
m)). With this configuration, it is possible to make the reflection of the incident light on the incident side and the reflection of the outgoing light on the exit side almost zero.

When wavelength conversion is performed using the wavelength conversion optical element, the Brewster angle of the cover material is set as follows according to the type of phase matching. (1) A case where a fundamental wave (ω) is incident on a wavelength conversion optical element to generate a second harmonic (2ω). The cover material c1 on the Type I incidence side has a Brewster angle corresponding to the o-polarized light, and the cover material c2 on the emission side has a Brewster angle corresponding to the e-polarized light. Type II The cover material c1 on the incident side has a Brewster angle corresponding to the polarization in the 45 ° direction so as to be able to support both the o-polarized light and the e-polarized light, and the cover material c2 on the output side is a Brewster angle corresponding to the e-polarized light. Star angle.

(2) When two different wavelengths of light are incident on the wavelength conversion optical element to generate a sum frequency light: The cover material c1 on the Type I incidence side has two different wavelengths (both polarization directions are o-polarized light). ) Is an angle approximating both Brewster angles corresponding to each of the above. The output side cover material c1 has a Brewster angle corresponding to e-polarized light. For example, the wavelength of CaF ₂ is 1064 nm, 532 nm, or 355 nm.
Brewster angles with respect to nm and 266 nm were 55.0 °, 55.14 ° and 55.3 as shown in Table 1 above.
3 °, 55.63 °, which is only about 1%. Therefore, if the Brewster angle is set to a value in the vicinity of 55 ° when the polarization direction is the same, it is possible to cope with light having the above-mentioned wavelength. When the Type II phase matching is Type II, one of the polarization directions of incident light is e-polarization and the other is o-polarization, and the polarization directions are different. For this reason, the inclined surface corresponding to both can not be formed in the cover material c1, and cannot be used for sum frequency generation.

[0023]

As described above, according to the present invention, an optical element made of a material having low hygroscopicity is bonded to the light incident surface and the light output surface of the wavelength conversion CLBO crystal, and the light is reflected on the surface of the optical element. Since the prevention film is formed or the inclined surface having a Brewster angle is formed on the optical element, it is possible to improve the moisture proof property and reduce the reflection loss on the light entrance / exit surface.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a wavelength conversion optical element according to a first embodiment of the present invention.

FIG. 2 shows a fundamental wave (ω) using the wavelength conversion optical element of FIG.
FIG. 4 is a diagram for explaining a case where a second harmonic (2ω) is obtained.

FIG. 3 shows a fundamental wave (ω) using the wavelength conversion optical element of FIG.
FIG. 4 is a diagram for explaining a case where a third harmonic (3ω) is obtained.

FIG. 4 is a diagram illustrating a configuration of a wavelength conversion optical element according to a second embodiment of the present invention.

FIG. 5 is a diagram illustrating a configuration of a wavelength conversion optical element according to a third embodiment of the present invention.

[Explanation of symbols]

 nc, nc1, nc2 CLBO crystal c1 to c4 Cover material Tf1 to Tf4 Antireflection film

Claims (3)

[Claims] 1. A flat optical element made of a material having low hygroscopicity is bonded to a light incident surface and a light output surface of a wavelength conversion CLBO crystal by an optical contact, and is bonded to a light incident side of the CLBO crystal. An anti-reflection film corresponding to the wavelength of incident light is provided on the surface of the flat optical element, and the anti-reflection film corresponding to at least the wavelength of the wavelength-converted light is provided on the surface of the flat optical element bonded to the light exit side of the CLBO crystal. An optical element for wavelength conversion, comprising a film. 2. A flat optical element made of a material having low hygroscopicity is bonded to a light incident surface of a wavelength conversion CLBO crystal by an optical contact, and antireflection corresponding to the wavelength of the incident light is made on the surface of the flat optical element. A film is provided, and an optical element made of a material having low hygroscopicity and having a Brewster angle corresponding to the wavelength and polarization direction of the wavelength-converted light is bonded to the light emitting surface of the CLBO crystal by optical contact. Wavelength conversion optical element. 3. An optical element made of a material having low hygroscopicity and having a Brewster angle corresponding to the wavelength and polarization direction of the incident light is bonded to the light incident surface of the wavelength conversion CLBO crystal by optical contact. Wavelength conversion, wherein an optical element made of a material having low hygroscopicity and having a Brewster angle corresponding to the wavelength and polarization direction of the wavelength-converted light is bonded to the light-emitting surface of the CLBO crystal by an optical contact. element.